The invention concerns a novel catalytic composition and a process for the dimerization, codimerization or oligomerization of olefins, in particular propylene. The composition comprises a mixture of at least one bivalent nickel complex containing two molecules of a tertiary phosphine, and at least one bivalent nickel compound or complex compound which does not contain phosphine. These mixtures are particularly for use in ionic liquid compositions as formed by quaternary ammonium halides and/or quaternary phosphonium halides with aluminium halides and/or alkylaluminium halides and optionally, aromatic hydrocarbons.
The nature of any substituents on the phosphorous atom of a phosphine which is bonded to nickel is known to have a considerable influence on enchainment in olefin molecules, in particular propylene, during catalytic dimerization as described by G Wilke et. al. in Ind. Eng. Chem., 1970, 62, no 12, p34, and in United Kingdom patent GB-A-1 410 430. G Wilke employed p-allyl complexes of nickel containing a single phosphine to catalyze dimerization. These complexes are difficult to prepare, relatively unstable and highly sensitive to water and to humidity. For this reason, the invention has never been put to practical use.
The phosphines were introduced into the catalyst by a variety of different methods. In particular, complexes formed by bivalent nickel salts with two equivalents of a tertiary phosphine were used in association with alkylaluminium halides as olefin dimerization catalysts, particularly for propylene. The advantage of using these complexes lies in their ease of preparation and their stability in air, also in their ease of use. The problem with these compounds is that they use two molecules of phosphine per nickel atom, and it has been shown that a single molecule is sufficient to have the desired effect. This is even more of a problem since alkylphosphines are expensive and, further, an excess of phosphine has a negative effect on the reaction rate.